1. Field of the Invention
The present invention relates to a three-group zoom lens and an image pickup apparatus, for example, a three-group zoom lens that is suitable for an electronic image pickup apparatus, such as a digital camera, a video camera, or the like.
Priority is claimed on Japanese Patent Applications Nos. 2004-136908, 2004-136909, and 2004-136910, filed Apr. 30, 2004, the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, digital cameras (electronic cameras) have received attention as the next generation of cameras, an alternative to silver-halide 35 mm-film cameras. Such digital cameras are broken down into several categories in a wide range from the high-end type for commercial use to the portable low-end type. In particular, cameras in the portable low-end type are desired to have high quality, such as having a low F number of about 2.8, a high zoom ratio of about 3, and a wide angle of view of about 60° while being thin.
The gravest bottleneck in diminishing the depth dimension of cameras is the thickness of an optical system, especially the zoom lens system from the surface located nearest to its object side to an image pickup plane. Recent technologies for slimming down cameras rely primarily on a collapsible lens mount that allows the optical system to be extended from a camera body for phototaking and be retracted thereto for carrying.
In order to achieve reduction in size and thickness, the reduction in the size of an image pickup element is required. For the reduced sized image pickup element to have the same number of pixels, the pitch between pixels should be reduced, and the resulting compromised sensitivity must be compensated by the optical system. In addition, diffraction may affect the sensitivity. Thus, an optical system having a smaller F number must be required.
Furthermore, to obtain a camera body to be thinner, it is effective to use the so-called rear focusing method for focusing in terms of the layout of the drive system rather than a method in which a front lens group is moved. Thus, an optical system that exhibits less fluctuation of aberration when the rear focusing method is employed must be selected.
For example, Japanese Unexamined Patent Application, First Publication No. 2002-372667 and Japanese Unexamined Patent Application, First Publication No. 2002-196240 disclose a negative lead type of zoom lens including three groups that has a low F number, a high zoom ratio of about 3, and a wide angle of view while being relatively compact.
Furthermore, Japanese Unexamined Patent Application, First Publication No. 2002-196240 and Japanese Unexamined Patent Application, First Publication No. 2003-222797 disclose a zoom lens that is suitable for used in the rear focusing method.
The above-described conventional zoom lenses have the following shortcomings.
In the techniques disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-372667 and Japanese Unexamined Patent Application, First Publication No. 2002-196240, off-axis aberrations cannot be completely eliminated by the first lens group and the second lens group, and the remainder of the aberrations is eliminated by providing a lens having a large amount of asphericity to the third lens group. Therefore, fluctuation of aberration becomes significant when the third lens group is moved, and accordingly it is difficult to obtain a sufficient image forming capability stably from the object point at infinity to an object point in a close range. Thus, the techniques are not suitable for the rear focusing method.
In addition, the technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-196240 and Japanese Unexamined Patent Application, First Publication No. 2003-222797, the shortest entire lens length in the wide-angle end or the telephoto end are large and the second lens group moves in a large amount upon varying magnifying power. Thus, the length of a lens-barrel portion that includes a cam mechanism for driving the second lens group is increased. Consequently, the overall length of the lenses is increased even when the lens is retracted, which may hinder size reduction.